The everyday interactions which we carry out with computers are constrained by our own perceptual and motor capabilities that are associated with the technology being used. To improve our overall experiences with both contemporary and emerging technological platforms there are two important steps which must be respected. 1) Understanding the nature of the low-level capabilities of the human user associated with the technology at use. 2) Designing interaction techniques and user interfaces based on an understanding of these capabilities. In this talk I will discuss my ongoing research which demonstrates how following this methodology can improve the human-computer dialogue. I will present the bubble cursor, a new selection technique for traditional graphical user interfaces which can significantly improve interface performance. Along with describing the design and implementation details of the technique, I will discuss a new probabilistic model for target acquisition which can be used to understand the bubble cursor's underlying theoretical value. I will then demonstrate how the above-mentioned methodology can also be applied to emerging technologies by discussing research which investigates interaction issues associated with true 3D volumetric displays. This includes formal experiments conducted to obtain an understanding of the human capabilities when interacting with volumetric displays, along with the design, implementation, and evaluation of new interfaces and interaction techniques that were guided by the insights gained from the earlier empirical studies.